Showing papers on "Reaction rate published in 1996"

Proton-transfer reaction dynamics

Abstract: In this article we discuss the progress made in understanding intermolecular and intermolecular reactions of proton (or hydrogen-atom) transfer. Femtosecond real-time probing, together with spectroscopic studies, in molecular beams are presented with selected examples of reactions. Reaction rates, tunneling dynamics and the nature of the reaction coordinate are examined and related to two-state multidimensional potential energy surfaces.

Ethylene Hydrogenation on Pt(111) Monitored in Situ at High Pressures Using Sum Frequency Generation

Abstract: Infrared−visible sum frequency generation (SFG) has been used to monitor the surface vibrational spectrum in situ during ethylene hydrogenation on Pt(111). Measurements were made near 1 atm of total pressure of ethylene and hydrogen and at 295 K. Kinetic information was obtained simultaneously with the surface vibrational spectroscopy by monitoring the reaction rate with gas chromatography. The macroscopic turnover rate and surface adsorbate concentration could then be correlated. During the reaction ethylidyne, di-σ-bonded ethylene, ethyl, and π-bonded ethylene were observed on the surface in various amounts depending on conditions. Ethylidyne, a spectator species during hydrogenation, competed directly for sites with di-σ-bonded ethylene and its surface concentration could be shown to be completely uncorrelated with the rate of hydrogenation. In contrast, π-bonded ethylene did not compete for sites with the ethylidyne overlayer and was observed on the surface regardless of the surface concentration of e...

Cross-Sections, Rate Constants and Transport Coefficients in Silane Plasma Chemistry

Abstract: This paper presents a critical review of the basic data concerning the physics and chemistry of low pressure SiH 4 glow discharges used to deposit hydrogenated amorphous silicon films (a-Si:H). Starting with an updated table of thermochemical data, we analyze the gas-phase elementary processes consisting of i) electron-molecule collisions, ii) ion-molecule collisions, iii) neutral-neutral collisions, iv) other electron and ion collisions involving electron-ion and ion-ion recombination, electron attachment on radicals and detachment of anions, and v) cluster growth kinetics in dusty plasmas. Experimental data or theoretical estimates are given and discussed in terms of cross-sections. collision and reaction rate constants, and transport coefficients. We also analyze the surface processes and reaction probabilities of ions, radicals and molecules.

Vertical distribution of Titan's atmospheric neutral constituents

Abstract: The vertical distribution of Titan's neutral atmosphere compounds is calculated from a new photochemical model extending from 40 to 1432 km. This model makes use of many updated reaction rates, and of the new scheme for methane photolysis proposed by Mordaunt et al. [1993]. The model also includes a realistic treatment of the dissociation of N 2 , of the deposition of water in the atmosphere from meteoritic ablation, and of condensation processes. The sensitivity of the results to the eddy diffusion coefficient profile is investigated. Fitting the methane thermospheric profile and the stratospheric abundance of the major hydrocarbons requires a methane stratospheric mixing ratio of 1.5-2% rather than 3%. Fitting the HCN stratospheric profile requires an eddy diffusion coefficient at 100-300 km that is 5-20 times larger than that necessary for the hydrocarbons. Most species are reasonably well reproduced, with the exception of CH 3 C 2 H and HC 3 N. The formation of CH 3 CN may involve the reaction of CN with either CH 4 or (preferably) C 2 H 6 . The observed CO 2 profile can be modeled by assuming an external source of water of ∼6 × 10 6 cm -2 s -1 . For a nominal CO mixing ratio of 5 × 10 -5 , the chemical loss of CO exceeds its production by ∼15%, and equilibrium is achieved for CO = 1 × 10 -5 .

Mechanism of carbon dioxide-catalyzed oxidation of tyrosine by peroxynitrite

Abstract: Peroxynitrite ion (ONO2-) reacted rapidly with CO2 to form a short-lived intermediate provisionally identified as the ONO2CO2- adduct. This adduct was more reactive in tyrosine oxidation than ONO2- itself and produced 3-nitrotyrosine and 3,3'-dityrosine as the major oxidation products. With tyrosine in excess, the rate of 3-nitrotyrosine formation was independent of the tyrosine concentration and was determined by the rate of formation of the ONO2CO2- adduct. The overall yield of oxidation products was also independent of the concentration of tyrosine and medium acidity; approximately 19% of the added ONO2- was converted to products under all reaction conditions. However, the 3-nitrotyrosine/3,3'-dityrosine product ratio depended upon the pH, tyrosine concentration, and absolute reaction rate. These data are in quantitative agreement with a reaction mechanism in which the one-electron oxidation of tyrosine by ONO2CO2- generates tyrosyl and NO2 radicals as intermediary species, but are inconsistent with mechanisms that invoke direct electrophilic attack on the tyrosine aromatic ring by the adduct. Based upon its reactivity characteristics, ONO2CO2- has a lifetime shorter than 3 ms and a redox potential in excess of 1 V, and oxidizes tyrosine with a bimolecular rate constant greater than 2 x 10(5) M-1 s-1. In comparison, in CO2-free solutions, oxidation of tyrosine by peroxynitrite was much slower and gave significantly lower yields (approximately 8%) of the same products. When tyrosine was the limiting reactant, 3,5-dinitrotyrosine was found among the reaction products of the CO2-catalyzed reaction, but this compound was not detected in the uncatalyzed reaction.

Fast one-phase oil-rich processes for the preparation of vegetable oil methyl esters

Abstract: A re-evaluation of kinetic data shows that the methoxide base-catalyzed methanolysis of soybean oil at 40°C (6:1 methanol:oil molar ratio) to form methyl esters proceeds approximately 15 times more slowly than butanolysis at 30°C. This is interpreted to be the result of a two-phase reaction in which methanolysis occurs only in the methanol phase. Low oil concentration in methanol causes the slow reaction rate; a slow dissolving rate of the oil in the methanol causes an initiation period. Intermediate mono- and diglycerides preferentially remain in the methanol, and react further, thus explaining the deviation from second-order kinetics. The same explanations apply for hydroxide ion catalyzed methanolysis. At the 6:1 methanol:oil molar ratio the addition of a cosolvent, such as 1.25 volumes of tetrahydrofuran (THF) per volume of methanol, produces an oil-dominant one-phase system in which methanolysis speeds up dramatically and occurs as fast as butanolysis. The critical separation of the glycerol-rich phase still occurs and does so faster than in the cosolvent-free system. For THF, recycle of solvent is simplified because of the similar boiling points of THF (67°C) and methanol (65°C). Possible explanations for the abnormal slowing of the methanolysis reactions are presented in terms of (1) lower rate constants for mono- and diglyceride reactions due to the formation of cyclic intermediates, (2) a fall in the polarity of the reaction mixture due to either methanol depletion or mixing of the oil, methanol and cosolvent, and (3) depletion of hydroxide ion when this is present.

Kinetics of the catalytic liquid-phase hydrogenation of aqueous nitrate solutions

Abstract: Liquid-phase reduction using a solid Pd/Cu bimetallic catalyst provides a potential technique for the removal of nitrates from waters. Kinetic measurements were performed for a wide range of reactant concentrations and reaction conditions in an isothermal semi-batch slurry reactor operating at atmospheric pressure. The effects of catalyst loading and initial nitrate concentration on the reaction rate were also investigated. The proposed intrinsic rate expression for nitrate disappearance is based on the conventional Langmuir-Hinshelwood kinetic approach, considering both equilibrium nitrate as well as dissociative hydrogen adsorption processes to different types of active sites, and assuming an irreversible bimolecular surface reaction between adsorbed reactant species to be the rate-controlling step. The apparent activation energy for catalytic liquid-phase nitrate reduction and the heat of nitrate adsorption, in the temperature range 280.5–293 K, were found to be 47 and 22 kJ/mol, respectively. It is confirmed that the process of catalytic liquid-phase hydrogenation of aqueous nitrate solutions undergoes a redox mechanism.

Thermal rate constants of the N 2 +O→NO+N reaction using ab initio 3 A″ and 3 A' potential energy surfaces

Abstract: Theoretical determinations of the thermal rate constants and product energy distributions of the N2+O→NO+N reaction, which plays a crucial role in hydrocarbon air combustion and high temperature air chemistry, are carried out using a quasiclassical trajectory method. An analytical fit of the lowest 3A′ potential energy surface of this reaction based on the CCI ab initio data is obtained. The trajectory study is done on this surface and an analytical 3A″ surface proposed by Gilibert et al. [J. Chem. Phys. 97, 5542 (1992)]. The thermal rate constants computed from 3000 to 20 000 K are in good agreement with the available experimental data. In addition, the dependence of the rate constant on the N2 internal state is studied. It is found that a low vibrational excitation can reduce the rate constant of this reaction by a factor of 3. Also, we investigate the effect of the N2 vibrational state on the product NO vibrational distribution, and it is found that at low N2 vibrational states, the NO vibrational dist...

Electrochemical Deposition of Very Smooth Lithium Using Nonaqueous Electrolytes Containing HF

Abstract: X-ray photoelectron spectroscopy and scanning electron microscopy methods were used for analysis of the surface layers of lithium deposited at various current densities from propylene carbonate containing 1.0 ml/dm{sup 3} LiClO{sub 4} and various amounts of HF, to investigate the effect of HF in electrolytes on the surface reaction of lithium during electrochemical deposition. The analyses indicate that the surface state of lithium and the morphology of lithium deposits are influenced by both the concentration of HF and the electrodeposition current. The first parameter for the electrodeposition of lithium is related to the chemical reaction rate of the lithium surface with HF and second to the electrodeposition rate of lithium. These results suggest that surface modification is effective in suppressing lithium dendrite formation when the chemical reaction rate with HF is greater than the electrochemical deposition rate of lithium.

OH Radical Reactions: The Major Removal Pathway for Polychlorinated Biphenyls from the Atmosphere

Abstract: Polychlorinated biphenyls (PCBs) are transported through the environment primarily in the atmosphere and may undergo chemical reactions, particularly with the OH radical, while in the vapor phase. Rate constants for the gas-phase reaction of 14 PCB congeners with the OH radical over the temperature range of 323−363 K were measured. The calculated temperature dependences of the reactions were used to estimate OH−PCB reaction rate constants at 298 K; see Table 4. These 298 K rate constants agree well with literature rate constants for PCBs containing zero to two chlorines and with OH−PCB reaction rate constants estimated from a structure−activity method. Calculated atmospheric lifetimes of PCBs due to OH-initiated reactions varied from 2 days for biphenyl to 34 days for a pentachlorobiphenyl. A simple model for the vertical concentration gradient of PCBs in the troposphere was developed and used to calculate the total global loss rate (8300 t yr-1) of PCBs from the atmosphere due to removal by OH. This path...

Self‐Assembly of Heterobimetallic Complexes and Reactive Nucleophiles: A General Strategy for the Activation of Asymmetric Reactions Promoted by Heterobimetallic Catalysts

Abstract: Heterobimetallic asymmetric catalysts, such as the lanthanum–lithium–binaphthol complex (La Li–BINOL), the aluminum–lithium–binaphthol complex (AlLi–BINOL), and a newly prepared gallium–sodium–binaphthol complex (Ga Na–BINOL), have been self-assembled with reactive nucleophiles, such as lithium nitronates and sodium malonates, to generate more efficient catalysts than the parent heterobimetallic catalysts. For example, by the combined use of La Li–BINOL (1 mol%; contains one H2O molecule) and BuLi (0.9 mol%) as the catalyst system, asymmetric nitroaldol reactions are greatly accelerated in all cases without a decrease in the optical purity of the nitroaldol products. Kinetic analyses have also been carried out on the Ga Na–BINOL-catalyzed Michael reaction of dibenzyl malonate with cyclohexenone, with or without NaOtBu. The calculated rate constants show that the combined use of Ga Na–BINOL and NaOtBu as the catalyst gives reaction rates that are about 50 times faster than with Ga Na–BINOL alone. This activation method should be useful for other asymmetric reactions catalyzed by heterobimetallic complexes.

Exact results for kinetics of catalytic reactions.

Abstract: The kinetics of an irreversible catalytic reaction on substrate of arbitrary dimension is examined. In the limit of infinitesimal reaction rate (reaction-controlled limit), we solve the dimer-dimer surface reaction model (or voter model) exactly in arbitrary dimension $D$. The density of reactive interfaces is found to exhibit a power law decay for $D<2$ and a slow logarithmic decay in two dimensions. We discuss the relevance of these results for the monomer-monomer surface reaction model.

Studies on single alkaline phosphatase molecules : reaction rate and activation energy of a reaction catalyzed by a single molecule and the effect of thermal denaturation : the death of an enzyme

Abstract: Single molecules of alkaline phosphatase are captured in a capillary filled with a fluorogenic substrate. During incubation, each enzyme molecule creates a pool of fluorescent product. After incubation, the product is swept through a high-sensitivity laser-induced fluorescence detector; the area of the peak provides a precise measure of the activity of each molecule. Three studies are performed on captured enzyme molecules. In the first study, replicate incubations are performed on the same molecule at constant temperature; the amount of product increases linearly with incubation time. Single enzyme molecules show a range of activity; the most active molecules have over a 10-fold higher activity than the least active molecules. In the second study, replicate incubations are performed on the same molecule at successively higher temperatures. The activation energy of the reaction catalyzed by a single molecule is determined with high precision. Single enzyme molecules show a range of activation energy; micr...

High pressure range of addition reactions of HO. II. Temperature and pressure dependence of the reaction HO+CO⇔HOCO→H+CO2

Abstract: Thermal rate constants of the complex‐forming bimolecular reaction HO+CO■HOCO→H+CO2 were measured between 90 and 830 K in the bath gas He over the pressure range 1–700 bar. In addition, the vibrational relaxation of HO in collisions with CO was studied between 300 and 800 K. HO was generated by laser photolysis and monitored by saturated laser‐induced fluorescence. The derived second‐order rate coefficients showed a pronounced pressure and complicated non‐Arrhenius temperature dependence. Above 650 K, the disappearance of HO followed a biexponential time law, indicating thermal instability of collisionally stabilized HOCO. By analyzing the corresponding results, an enthalpy of formation of HOCO of ΔHof,0=−(205±10) kJ mol−1 was derived. On the basis of energy‐ and angular‐momentum‐dependent rates of HOCO formation, activated complex properties for the addition reaction HO+CO→HOCO were derived from the limiting high‐pressure rate constants; with the limiting low‐pressure rate constants, activated complex pr...

Kinetics of acetic acid esterification over ion exchange catalysts

Abstract: Computer simulation showed that catalytic distillation is an attractive process for the removal of dilute acetic acid from wastewater. Selection of catalysts and kinetic data have been obtained for the design of the catalytic distillation column. Kinetic measurements were conducted in a batch reactor. Methanol was added to the dilute acetic acid solutions and reacted with the acid in water to form methyl acetate and water. The reaction can be catalyzed by solid acid catalysts. It was found that Amberlyst 15 was an effective catalyst for this reaction. The effects of stirrer speed, reaction temperature, reactant concentration and catalyst loading on reaction rate were investigated. A complete kinetic equation for describing the reaction catalyzed by Amberlyst 15 was developed. This equation can be used in the simulation and design of the catalytic distillation column for removing acetic acid from wastewater.

Impact of Catalyst Metal−Acid Balance in n-Hexadecane Hydroisomerization and Hydrocracking

Abstract: The reaction pathways and kinetics of n-hexadecane hydroisomerization and hydrocracking were determined in the presence of each of three platinum-containing dual-function catalysts: (a) Pt on a proprietary zeolite (Pt/Ζ), (b) Pt on silica−alumina (Pt/Si−Al), and (c) Pt on MCM-41 (Pt/MCM-41). The reaction networks were used to interpret differences in isomerization selectivity. The low isomerization selectivity observed in the presence of Pt/Si−Al was shown to be a consequence of changes in both relative isomerization/cracking rates and reaction pathways. Using the classical bifunctional reaction scheme, the changes in pathway were hypothesized to be consistent with changes in the relative concentrations of metal and acid sites (i.e., the metal−acid balance). On the basis of a recently proposed model of dual-function catalysis, the different observed pathways were subsequently shown to be those expected in two limiting case of the metal−acid balance. The simplified quantitative picture given here provides...

Stacking Faults in β-SiC Formed during Carbothermal Reduction of SiO2

Abstract: Factors affecting stacking fault formation during the synthetic fabrication of {beta}-SiC were investigated in the present study. Two main reaction routes in the carbothermal reduction of SiO{sub 2}--solid-solid and solid-gas reactions--led to the formation of {beta}-SiC, depending on the vapor pressure of the SiO gas. The {beta}-SiC formed by the solid-gas (carbon-SiO gas) reaction showed a whisker morphology with a high stacking fault density (SFD), whereas that formed by the solid-solid (Si-carbon) reaction exhibited spherical particles with a low SFD. The average size of the synthesized particles decreased with decreasing reaction temperature and time, and their stacking fault content, measured by X-ray diffraction, was higher than the true value, possibly because of a size effect. The stacking fault density increased with increase in the heating rate because of an increased reaction rate.

Quantum mechanical three-dimensional wavepacket study of the Li+HF --> LiF+H reaction

Abstract: A three‐dimensional time‐dependent quantum mechanical wavepacket method is used to calculate the state‐to‐state reaction probabilities at zero total angular momentum for the Li + HF → LiF +H reaction. Reaction probabilities starting from several different initial HF vibrational–rotational states (v=0,j=0,1,2) and going to all possible open channels are computed over a wide range of energies. A single computation of the wavepacket dynamics yields reaction probabilities from a specific initial quantum state of the reactants to all possible final states over a wide range of energies. The energy dependence of the reaction probabilities shows a broad background structure on which resonances of varying widths are superimposed. Sharp resonance features seem to dominate particularly at low product translational energies. There are marked changes in the energy dependence of the reaction probabilities for different initial or final diatom rotational quantum numbers, but it is noticeable that, for both reactants and...

Mathematical Modeling of Char Reactivity in Ar−O2 and CO2−O2 Mixtures

Abstract: The kinetics of the coal char + O2 reaction was studied by thermogravimetry. Low sample masses were employed to ensure an approximate kinetic regime. Special emphasis was placed on clarifying how the recirculation of the flue gases (i.e. the presence of a high amount of CO2 at low O2 concentrations) affects the reactivity. The ambient gas concentrations varied from 100% O2 to 5% O2 in CO2 or Ar. A semi-empirical model is presented which can approximate the reactivity changes during the conversion and takes into account the heterogeneity of the samples. A least squares evaluation procedure resulted in a good fit to the experimental data over a wide variety of temperature programs and ambient gas concentrations. The overall burn-off time of the samples varied from eight minutes to three hours depending on the experimental conditions. The reaction rate was found to be proportional to the O2 concentration of the ambient gas and was not influenced by the presence of high amounts of CO2. The reaction started with a sharp acceleration period indicating an initial activation of the char surface.

Quantitating the Balance between Enthalpic and Entropic Forces in Alkanethiol/Gold Monolayer Self Assembly

Abstract: We have studied the temperature and adsorbate concentration dependence of alkanethiol/gold monolayer formation in situ and in real time using a quartz crystal microbalance to monitor the rate of reaction. The temperature dependence of the experimental rate constants, for a given concentration, demonstrates that the formation of these monolayers is not diffusion rate limited for the conditions we report here. Our data are modeled accurately by the Langmuir adsorption isotherm and, using this model, we have determined the adsorption and desorption rate constants for monolayer self assembly. We have extracted from these data the thermodynamic quantities ΔGads, ΔHads, and ΔSads for monolayer formation. For 1-octadecanethiol adsorption from n-hexane onto gold, ΔGads is temperature dependent and is ca. −5.5 kcal/mol, ΔHads = −20 ± 1 kcal/mol, and ΔSads = −48 ± 1 cal/(mol·K). These data have direct bearing on the broader field of interface chemistry because they underscore the dominant role that entropy plays in...